1. Field of Invention
This invention relates to zero-voltage or zero-current switching (xe2x80x9csoft switchingxe2x80x9d) of DC-to-DC converters comprising at least a pair of switching devices.
2. Description of Related Art
DC-to-DC converters are used to obtain a specified DC voltage in a power circuit. Noise and power losses occur in DC-to-DC converters when turning-on and turning-off the switching devices. The soft switching technique is known, so far, for decreasing noise and power losses. Resonant DC-to-DC converter and inductor commutating circuits, etc. are proposed as a means to achieve soft switching.
For instance, Japanese Patent Unexamined Publication 7-46853 discloses a half-bridge soft switching inverter and controlling method thereof. Soft switching inverter 64 of the publication is shown in FIG. 10. A pair of switching devices Q1 and Q2 are connected in series between +E0 volt and xe2x88x92E0 volt. Output filter 22 is connected to output node a. Load (ZL) 24 is connected to output node b of output filter 22. Capacitors C1, C2, and diodes D1, D2 are connected to the switching devices Q1 and Q2 in parallel at both ends of Q1 and Q2, respectively. Output filter 22 is composed of inductor LF, connected between output node a and output node b, and capacitor CF connected between output node b and ground. Load 24 is connected between output node b and ground. Inverter controller 64 can output an objective DC voltage between +E0 and xe2x88x92E0 volt, as an output voltage, by switching the switching devices Q1 and Q2 in a timely manner. However, direct current flows from +E0 to xe2x88x92E0, when the both switching devices Q1 and Q2 turn on at the same time. As a result, power loss becomes very large, and the switching devices Q1 and Q2 suffer damages.
In an attempt to overcome the problems described above, it is generally known to have a dead time, that turns off both the switching devices Q1 and Q2 simultaneously, in switching the Q1 and Q2.
Output voltage of output node a of the inverter controller 64, having an ideal dead time, is shown in FIG. 5. The voltage of output node a changes by switching of Q1, Q2, and resonating of the inductor LF and the capacitor CF. The output voltage of output node a for the period of dead time changes along with the electrical charge and discharge of capacitors C1 and C2 connected with Q1 and Q2 in parallel, respectively. These voltage changes are very rapid compared with the voltage changes in the period when Q1 or Q2 is turned on.
FIG. 6 shows voltages of switching devices Q1 and Q2, output voltage VSx of the node a, flowing currents in Q1 and Q2 and the time differentiated signal output voltage dVSx/dt, when the dead time A is shorter than an ideal period. At the moment Q1 turns on earlier than the ideal dead time A, the output node a is pulled up rapidly. As a result, an excessive current momentarily flows in Q1.
On the contrary, when the dead time A is longer than the ideal period as shown in FIG. 7 and Q1 remains turned-off longer than the ideal period, the output voltage VSx of node a becomes higher than the power-supply voltage. Afterwards, at the moment Q1 turns on, the output voltage of node a is pulled down rapidly. As a result, an excessive current momentarily flows in Q1.
A similar phenomenon occurs on the switching device Q2 as shown in FIGS. 8 and 9.
Japanese Patent Unexamined Publication 7-46853 discloses that when the output voltage of node a becomes equal to a certain set reference voltage, then the switching device Q1 or Q2 is turned on. Therefore, switching noise and switching power losses can be decreased by above mentioned zero voltage switching.
However, when there is a rapid pull-up and rapid pull-down in the current value flowing in the switching devices Q1 and Q2 as shown in FIGS. 6 through 9, it is very difficult to determine when the output voltage of node a is equal to the set reference voltage. Also, it is difficult to measure the output voltage with high accuracy, and with high speed. Therefore, it was very difficult to achieve the teaching of 7-46853, realistically.
This invention provides a soft switching DC-to-DC converter usable to reduce noise and power losses attributed to the turning on and turning off of switching devices. The present invention substantially eliminates the over current flow attributed to a switching device turning on before the end of the dead time period, or a switching device remaining off beyond the dead time period. This invention separately provides a method for controlling a DC-to-DC converter.
In various exemplary embodiments the DC-to-DC converter includes a switching circuit, error amplifier, pulse-width modulator, differentiator and dead time adjusting circuit. The error amplifier compares a voltage output to a reference voltage, and generates a comparison result. The pulse-width modulator adjusts the output pulse in accordance with the comparison result from the error amplifier. The differentiator outputs a differentiated signal dVSx/dt of the output voltage VSx of the switching circuit. The differentiated signal dVSx/dt outputs a signal that indicates whether there has been an over current in either the positive or negative direction. Typically, the over current flow will generate a peak in the differentiated signal. The dead time adjusting circuit adjusts the dead time period when it detects a spike or peak in the differentiated signal dVSx/dt to obtain the appropriate dead time period.
These and other features and advantages of this invention are described in, or apparent from, the following detailed description of the devices and methods according to this invention.